docs: def ndef(ctx, slot, prefix):
      """
      Select slot configuration to use for NDEF.
  
      The default prefix will be used if no prefix is specified.
      """
      dev = ctx.obj['dev']
      controller = ctx.obj['controller']
      if not dev.config.nfc_supported:
          ctx.fail('NFC interface not available.')
  
      if not controller.slot_status[slot - 1]:
          ctx.fail('Slot {} is empty.'.format(slot))
  
      try:
          if prefix:
              controller.configure_ndef_slot(slot, prefix)
          else:
              controller.configure_ndef_slot(slot)
      except YkpersError as e:
          _failed_to_write_msg(ctx, e)